This invention relates to a system and method for determining the dip of subsurface formations penetrated by a borehole. More particularly, this invention is directed to an apparatus and method that uses electromagnetic energy to determine a dip of subsurface formations penetrated by a borehole.
A discussion of the determination of dip of subsurface formations and of dipmeters is found in FORMATION EVALUATION by Edward J. Lynch, Harper & Row, Publishers, New York, Evanston and London, beginning at page 341. It is there pointed out that the problem of the determination of formation dip evolves into one of locating three points within a depositional layer with reference to a horizontal plane such that a plane is defined by the three points and the angle of dip is that angle between this plane and the horizontal plane. Instruments have been used which can record three electric logs properly spaced and oriented in a single borehole to determine the dip of the subsurface formations penetrated by the borehole. These instruments require an accurate directional survey of the borehole. Wireline dipmeters which are commonly used for this purpose must make three basic measurements. The first is the measurement of dip of the formation relative to the borehole. This has been done by including identical sets of electrodes spaced at 120.degree. and all on the same plane perpendicular to the axis of the tool. The second is a measurement of the direction and angle of inclination of the borehole, and the third is a measurement of the orientation of the tool relative to magnetic north.
Early dipmeters used three SP curves to get the dip relative to the hole axis and used a photoclinometer to determine the hole inclination and direction. A continuous type of dipmeter later introduced employs three microlog devices to make the three required eletrical logs. The microdevices are always pressed against the wall of the borehole, thus causing the spacing between them to change as the size of the borehole changes. This necessitates the recording of the borehole size.
In U.S. Pat. No. 3,388,323 to Allen A. Stripling, there is described a technique which has become known as "induction logging" for determining a dip of subsurface formations. Two independent parameters indicative of magnetic susceptibility and electrical conductivity or resistivity are derived from magnetic field measurements carried out at three angular positions adjacent a borehole wall. Three coils are employed for carrying out the borehole measurements. These coils are energized to generate magnetic fields at three angular regions around the borehole wall, and the outputs thereof are phase separated into magnetic susceptibility and resistivity measurements. A 10,000-cycle-per-second oscillator and a 1,000-cyle-per-second oscillator are employed for energizing the coils. These high- and low-frequency oscillators are employed to increase the sensitivity of the coil to changes in electrical conductivity and magnetic susceptibility.
In a paper entitled "ELECTROMAGNETIC PROPAGATION . . . A New Dimension in Logging" by Thomas J. Calvert, Rama N. Rau, and Larry E. Wells, prepared for presentation at the 1977 47th Annual California Regional Meeting of the Society of Petroleum Engineers of AIME in Bakersfield, Calif., Apr. 13-15, 1977, there is described the operating principles of a well logging tool that measures the phase shift and attenuation of microwave-frequency energy propagated through the formations near a borehole. Also described is an interpretation method for deriving from these measurements the values of porosity and water saturation of earth formations. It is pointed out that this tool is most accurate in fresh water. It is further pointed out at page 15 that the tool is designed for fresh mud applications and that signal levels may be too low in salt muds and signals are not reliable in oil- or gas-filled boreholes.
Logging techniques using electromagnetic energy are described in U.S. Pat. Nos. 3,849,721 to Thomas J. Calvert; 3,944,910 to Rama N. Rau; 4,107,597 to Meadow et al; and 4,383,220 to Baldwin. In the Baldwin patent a borehole tool is provided that is adapted for moving through the borehole and that has at least three antenna arrays positioned approximately equally around the longitudinal axis of the tool. Each antenna array has one transmitting antenna and one receiving antenna spaced longitudinally one from the other. The tool is adapted for supporting and positioning the antenna arrays against the wall of the borehole. The tool further includes means for simultaneously supplying electromagnetic energy into the earth formations. Further provided are means coupled to each of the receiving antennas for detecting the signal received by each receiving antenna.